Image sensors are used in a variety of digital image capture systems, including products such as scanners, copiers, and digital cameras. The image sensor is typically composed of an array of light-sensitive pixel cells that are electrically responsive to incident light reflected from an object or scene whose image is to be captured.
A CMOS imager includes a focal plane array of pixel cells, each cell includes a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. In a CMOS imager, the active elements of a pixel cell, for example a four transistor (4T) pixel cell, perform the necessary functions of (1) photon to charge conversion; (2) resetting a floating diffusion region to a known state; (3) transfer of charge to the floating diffusion region; (4) selection of a pixel cell for readout; and (5) output and amplification of a signal representing a reset voltage and a pixel signal voltage based on the photo-converted charges. The charge at the floating diffusion region is converted to a pixel or reset output voltage by a source follower output transistor.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, all assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are hereby incorporated by reference herein in their entirety.
A conventional CMOS four-transistor (4T) pixel cell 10 is illustrated in FIGS. 1A. The illustrated cell 10 includes a pinned photodiode 13 as a photosensor. Alternatively, the CMOS cell 10 may include a photogate, photoconductor or other photon-to-charge converting device, in lieu of the pinned photodiode 13, as the initial accumulating area for photo-generated charge. The photodiode 13 includes a p+ surface accumulation layer and an underlying n- charge accumulation region formed in a p-type semiconductor substrate. In the exemplary pixel 10, accumulated charges are accumulated electrons.
The pixel cell 10 has a transfer gate 7, which is part of a transfer transistor 8, for transferring photocharges generated in the n- accumulation region to a floating diffusion region 3. The floating diffusion region 3 is further connected to a gate 27 of a source follower transistor 28. The source follower transistor 28 provides an output signal to a row select transistor 38 having a gate 37 for selectively gating the output signal to a column line 50. The column line 50 is selected for readout by a column select transistor 52, which applies a current source 54 to column line 50. A reset transistor 18 having a gate 17 resets the floating diffusion region 3 to a specified charge level by connecting it to a supply voltage Vaa-pix before each charge transfer from the n- accumulation region of the photodiode 13.
FIG. 1B illustrates a block diagram of an exemplary CMOS imager 108 having a pixel array 140 comprising a plurality of pixel cells arranged in a predetermined number of columns and rows, with each pixel cell being constructed as illustrated and described above with respect to FIGS. 1A or using other known pixel architectures. Attached to the array 140 is signal processing circuitry for controlling the pixel array 140, as described herein, at least part of which may be formed in the substrate. The pixel cells of each row in array 140 are all turned on at the same time by a row select line, and the pixel cells of each column are selectively output by respective column select lines. A plurality of TX, read, row select and column select lines are provided for the entire array 140. The row lines are selectively activated by a row driver 145 in response to row address decoder 155. The column select lines are selectively activated by a column driver 160 in response to column address decoder 170. Thus, a row and column address is provided for each pixel cell.
The CMOS imager 108 is operated by a timing and control circuit 150, which controls address decoders 155, 170 for selecting the appropriate row and column lines for pixel readout. The control circuit 150 also controls the row and column driver circuitry 145, 160 such that they apply driving voltages to the drive transistors of the selected row and column lines. The pixel column signals, which typically include a pixel reset signal Vrst and a pixel image signal Vsig, are output to column driver 160, on output lines, and are read by a sample and hold circuit 161. Vrst is read from a pixel cell 10 immediately after the floating diffusion region 3 is reset. Vsig represents the amount of charges generated by the photosensitive element of the pixel cell 10 in response to applied light during an integration period. A differential signal (Vrst−Vsig) is produced by differential amplifier 162 for each readout pixel cell. The differential signal is digitized by an analog-to-digital converter 175 (ADC). The analog to digital converter 175 supplies the digitized pixel signals to an image processor 180, which forms and outputs a digital image.
Blooming occurs when a photodiode or other photosensitive device is overexposed, i.e., the photodiode 13 is exposed to more photons than it is capable of absorbing and converting to electrons. This is typically due to exposure of a pixel to an overly intense light source, or to exposing the pixel for too long a period of time. The excess electrons can spill over into pixels in nearby rows and columns. This overflow distorts the image, and can wash out portions of the image near the overexposed portion.
A typical technique for exposing and reading out images using pixel array 140 is by using a rolling shutter. That is, as row N is being read out, row N+X is hard reset, with X representing a number of rows further down in the array 140. A hard reset means that both the reset transistors and transfer transistors are turned on to reset the photodiode, and when the transfer transistor is turned off, an integration period begins. The time difference between a hard reset of a row and its subsequent readout is the pixel exposure time, also known as integration time.
Referring now to FIGS. 2A-2F, the operation of a conventional rolling shutter on a portion of a conventional CMOS pixel array 140 is now described. The rolling shutter, operated by control circuit 150 (FIG. 1B) includes a read row 200 and a hard reset row 201. An overexposed pixel 210 and bloom region 211 is also shown. For the purposes of this example, the integration period X of the rolling shutter is the time it takes to sequentially read out 10 rows. The integration period can be set responsive to the amount of incident light and on the capacity of the photodiodes 13 in order to prevent individual pixels from overexposing and blooming during the integration period.
Referring now to FIG. 2A, as read row 200 (at row 6) is being read out, hard reset row 201 (at row 16) is hard reset, which then begins an integration period for row 16 where row 16 begins to collect photons until the read row 200 reaches row 16. FIGS. 2B-2F show the row by row progression of read row 200 and hard reset row 201, with the hard reset row 201 always 10 rows ahead of the read row 200. Once rows 6-15 are read out in sequence, row 16 will then be read out, and so on. This conventional rolling shutter technique does not adequately compensate for blooming, however.
Blooming can often affect pixels several rows or columns away from the initially overexposed pixel. In many cases, the adjacent pixels, which absorb a large portion of the deflected electrons from the initial overexposed pixel, can be overexposed themselves and begin to deflect electrons. Additionally, many electrons may be deflected to pixels several rows or columns away.
This blooming problem is exacerbated when, as is often the case in overexposure situations, large groups of adjacent pixels are exposed to intense light and become overexposed. The cumulative effect of the overexposed pixels can accelerate the displacement of extra electrons, overexposing and distorting pixels many rows and columns away from the initially overexposed pixels in a very short amount of time.
Conventional rolling shutters cannot fully compensate for this phenomenon. Referring again to FIGS. 2A-2F, an overexposed pixel 210 having a bloom region 211 is shown. For the purposes of this example, a single pixel 210 is exposed to light of sufficient intensity to overload the photosensor charge accumulation region 14 (see FIG. 1A) and send excess electrons as far away as five pixels in all directions. Thus, all pixels within the bloom region 211, having a radius of approximately five pixels, will produce an inaccurate result because of the excess electrons from pixel 210.
Referring again to FIG. 2A, when hard reset row 201 reaches and discharges row 16, i.e., hard resets the pixels in the row), the effects of the overexposed pixel 210 on the portion of the row in the bloom region 211 are temporarily eliminated. However, as shown in FIG. 2B, the blooming effect on a portion of row 16 can reemerge before the end of the integration period for row 16. A modern pixel array can have in excess of 1000 rows; therefore, in this example where the integration time is 10 rows, an overexposed pixel 210 can deflect electrons to nearby pixels for the time it takes to read out 990 rows, or longer. Also, because electrons can be deflected farther than the dimensions of an individual pixel, the hard resetting of row 17 will not prevent photon overflow to still integrating row 16. Also, in situations where the light is particularly intense, or where the photon capacity of a pixel cell is particularly small, the portion of row 17 in the bloom region 211 may refill and overflow to row 16 in less time than it takes to integrate one row, further distorting the portion of row 16 in the bloom region 211.
FIGS. 2C-2F show the progression of the rolling shutter and its inability to fully compensate for blooming. The hard reset row 201 must reach the overexposed pixel 210 (at row 21) to fully discharge the excess electrons by a hard reset, as shown in FIG. 2F. Before the hard reset row 201 reaches the overexposed pixel 210 at row 21, however, portions of rows 16-20 in the bloom region 211 will have already been distorted by the excess electrons which have been deflected from the pixel 210 before hard reset, as shown in FIGS. 2B-2E. Although overexposed pixel 210 is hard reset and stops contributing to the distortion of bloom region 211, the effects of the excess electrons in bloom region 211 will remain until the row is discharged, either by being hard reset for rows 22+, or by being read out for rows 16-20, distorting the portion of the image in that portion of the bloom region 211.
There exists a need and desire for an improved rolling shutter to reduce blooming in CMOS pixel arrays caused by intense light sources and by pixel cells having small charge accumulation capacity.